The present invention pertains to coherent transponders for a communications system and especially to transponders utilized in satellites and the like. Generally, communication with the satellite through a transponder is utilized for determining the relative position of the satellite and may, in addition, be utilized for messages and other information. Basically, the transponder includes a receiver/transmitter carrier tracking loop which must track and demodulate an RF signal transmitted to the satellite carrying the transponder and transmit back to earth a signal which is coherently related by a precise frequency ratio. Traditional methods for accomplishing the tracking and coherent turn-around functions employ analog phase detectors, loop filters, voltage controlled oscillators and an assortment of frequency multipliers for deriving receiver local oscillator signals and the transmitter output. Generally, these analog components require large amounts of space and power. Further, narrow band local oscillator chain elements such as step recovery diode multipliers are costly to align.
In the prior method the receiver LO multiplier chain and the transmitter multiplier chain had to be implemented separately to allow a common multiple (VCXO at 2F1) to act as the coherent reference. This scheme normally requires at least four step recovery diode multipliers and stringent filtering requirements.